This invention relates to valves, valve packing, and methods of valve packing replacement for turbines.
Gas turbines utilize a plurality of valves to control the amount of fuel being burned by the turbine. These valves must be packed to seal against fuel leaks. Because of the environmental regulations presently applicable to these valves, any leak through the packing of the valve must be repaired.
A typical valve comprises a valve shaft which extends through a stuffing box which houses the packing for the valve. One end of the valve shaft is external to the valve and is operated on by an actuator while the other end of the valve shaft is internal to the valve and operates to open and close the valve. The packing for the valve typically includes a compression member, usually a coiled spring with an aperture therethrough large enough to receive the valve shaft. The spring is placed in the stuffing box toward the internal end of the valve shaft. Thus, the valve shaft extends through the aperture of the compression member, and the compression member is inserted into the stuffing box toward the valve end of the valve shaft. Following the spring, three TEFLON.RTM. packing rings are placed over the valve shaft and pushed up into the stuffing box. Next, a conventional lantern ring with a central aperture is placed over the valve shaft and inserted up into the stuffing box. Finally, a packing gland having a centrally located aperture is placed over the valve shaft. The packing gland has a flange extending radially outward from the valve shaft for attachment to the valve and a flange extending axially into the stuffing box, so that when the packing gland is attached to the valve case, the spring is compressed thereby compressing the packing rings and expanding the packing rings radially outward and radially inward to create a seal between the valve shaft and the stuffing box wall. This packing assembly allows the valve shaft to translate longitudinally through the stuffing box and is designed to prevent fuel leaks. A sealing ring is provided between the lantern ring and the packing gland to prevent external contaminants from being introduced into the stuffing box and damaging the packing.
The turbine fuel valves operate at temperatures around 300.degree. F. and pressures ranging from 120 psig. to 330 psig. Under these conditions the valve packing described has failed frequently creating environmental compliance problems.
Because the cabinet in which the valves are housed does not provide enough space for the valve shaft to be pulled far enough out of the valve to remove the valve packing assembly from the stuffing box, it is necessary to dismantle the valve to repair the valve packing. Thus, the valve must be bypassed and completely disassembled to remove the packing rings and the sealing ring. Because the valves must be bypassed and completely dismantled, the repair takes approximately 12-16 hours. Further, because the sealing ring is interposed directly between the packing gland and lantern ring, it is in direct contact with the stuffing box wall, and thus, it must be scraped off the stuffing box wall before the new packing is placed into the stuffing box. Again because the cabinet in which the valves are housed provides limited work space, this is a time consuming and difficult task. Therefore, valve packing assembly repair is time consuming and costly.
The time required to complete valve packing changes increases the cost of running the turbine. Further, the frequency with which the valve packing is repaired adds to the cost of running the turbine. It is desirable, therefore, to reduce the amount of time necessary to repair the valve packing and to reduce the frequency with which the valve packing fails.